1. Technical Field
The present invention relates to systems and methods for minimizing co-channel interference in signals sent to or received from a satellite, and more particularly, to systems and methods for reducing the co-channel interference on-board a satellite.
2. Description of the Related Art
It is commonplace for communications satellites to receive and transmit signals from and to ground stations in a number of different beam coverage areas, while utilizing the same uplink and/or downlink frequencies. Such frequency reuse schemes (e.g., using different polarization or spatial/geographic isolation) increase the overall satellite capacity, as measured by the throughput data rate in bits per second, but also introduce co-channel interference between the signals in the various beams.
The co-channel interference is evident between beams with identical polarization but it is also present between beams with opposite polarization (cross-polarization). The co-channel interference varies with the number of beams that share the same frequency band and the isolation between beams, as well as the relative location of transmit and receive earth stations with reference to the centers of the corresponding satellite beams. The co-channel interference also depends on the relative power between the carriers transmitted in the different beams.
In the related art, the antennas of the traditional communications satellites are designed in such a way that co-channel interference is in the order of 22 dB, thereby allowing the operation of carriers with signal to noise ratios (SNR) of up to 20 dB. Two methods are used to achieve this goal: ‘Spatial Isolation’ where, the beams serve different geographical regions as shown in FIG. 1A; and ‘Polarization Isolation’ where beams that could serve similar areas uses opposite polarization as shown in FIG. 1B. In the first method, in general, the isolation between beam centers is better than 27 dB. In the second method the polarization isolation of the satellite and the terrestrial antennas plays an important role minimizing the co-channel interference. In general the polarization isolation of the satellite antennas is better than 27 dB while the polarization isolation of the terrestrial antennas varies but it is recommended to be better than 26 dB.
If the isolation between beams is reduced due to a reduction of the geographical distance between beams or use of satellite antennas with less isolation, the corresponding SNR is likely to be reduced due to the increase in co-channel interferance. Consequently, either the link quality is degraded or the throughput (typically expressed in bits per second, bps) transmitted through the satellite link is reduced. In other words, the bandwidth efficiency is reduced (bandwidth efficiency typically expressed in bps per Hertz, bps/Hz).
As an example of geographic/spatial frequency reuse in related art satellites, one beam might cover the Western Hemisphere (WH, e.g., the Americas) and another beam, using the same frequency range might cover the Eastern Hemisphere (EH e.g., Europe and Africa), a shown in FIG. 1A. As shown in FIG. 1A, the frequency plan in this scenario uses the same frequency and same frequency bands of 72 MHz as well as the same polarization (Pol. A) for both beams for up-link and down-link signals, since the beams are spatially isolated. An example of the polarization isolation frequency reuse in related art satellites is shown in FIG. 1B. As shown, the beam for the North East Zone (NEZ) serves an area similar to the beam for the Eastern Hemisphere (EH). Therefore, as shown in the frequency plan, both beams use the same frequency and same frequency bands of 72 MHz but use opposite polarization for up-link and down-link signals. In particular, the beam for the EH uses Pol. A, whereas the beam for the NEZ uses Pol. B.
When beams use linear polarization covering similar areas, the polarization isolation is achieved by using horizontal and vertical polarizations on each respective beam. Instead, if the beams use circular polarization, the polarization isolation is achieved using right and left hand polarizations.
Although FIGS. 1A and 1B show traditional bent-pipe satellites implementing “Spatial Isolation” and “Polarization Isolation” to minimize co-channel interference, the “Spatial Isolation” and “Polarization Isolation” techniques could also be implemented in on-board processing satellite. As understood by a skilled artisan, the traditional bent-pipe satellites receive signals from earth station, amplify the received signals and if necessary, shift the uplink frequency to a downlink frequency, and transmit the amplified (and possibly shifted) signal to an earth station. On the other hand, in-board processing satellites, the received signal is demodulated, decoded, re-encoded, and modulated before being transmitted to an earth station.
The ability to advantageously exploit geographic (spatial) frequency re-use is limited by various factors including the distance between the beam centers, the beam gain roll off with distance from the center of one beam towards another co-frequency beam, and the number of co-frequency beams. These factors are a function of the satellite antenna design and quality. An additional factor is the level difference of the signals transmitted in the different beams from the earth stations (uplink case) or from the satellite (downlink case, also referred to below as down-link case). As an example consider the case where two earth stations transmitting on the same frequency, one in Cape Town transmitting to the Eastern Hemisphere Beam and the other in New York transmitting to the Western Hemisphere Beam. If the isolation between the two beams for the stations at New York and Cape Town is 26 dB and the two stations are transmitting at the same level, the uplink co-channel interference ratio would be 26 dB. However, if the New York station were to increase its uplink level by 6 dB then the effective uplink co-channel isolation at Cape Town would be reduced by 6 dB to 20 dB resulting in a co-channel interference ratio of 20 dB. In related art satellite designs there could be 4 to 6 beams that are co-channel or beams reusing the same frequency band. So the uplink levels of earth stations transmitting to the additional two to four beams would have to be taken into consideration in this example.
In the newer related art satellite designs employing multiple circular spot beams (called multi-spot) as shown in FIG. 2, there can be forty or more beams, on the order of ten of which are co-channel, covering areas with diameters of only a few hundred miles. FIG. 2 illustrates four circular spot beams 1-4. As shown in the frequency plan, all four beams 1-4 use the same frequency and same frequency bands of 250 MHz for up-link and down-link signals, but beams 1 and 3 use Pol. A, whereas beams 2 and 4 use Pol. B to achieve polarization isolation. For these satellites the beam isolation provided by the satellite antenna alone may not always be sufficient to obtain the desired beam to beam isolation—In addition, uplink level differences for earth stations transmitting in nine or more beams must be considered.
The spatial isolation and the polarization isolation methods utilized in the traditional satellites that support open networks architecture to minimize the impact of the co-channel interference limit the number of beams, and as a consequence limit the total satellite throughput capacity. In general, a traditional satellite will reuse frequencies and polarizations two or three times to achieve a total useful bandwidth between 2 and 3 GHz. Some satellites that support closed-network architecture use a large number of beams (multi-beams) but there is an associated network management system that assigns the carriers' frequencies, interleaving them to reduce the co-channel interference. This solution also limits the total satellite capacity because, in this case, when one carrier operates at a given frequency in one beam other carriers at the same frequency in the co-channel beams are not allowed to operate. As a consequence, some of the satellite resources need to be available all the time but are rarely used. This type of satellite typically has 20 GHz of equipped bandwidth, but the useful bandwidth is reduced to 12 or 14 GHz at a given time due to interleaved managed operation.
The discussion so far has focused mainly on the uplink case. The same considerations apply to the downlink; when co-channel interference is introduced in the downlink signals, i.e., the signals transmitted from the satellite to the ground stations, the achievable received SNR at the receiver ground station for a given downlink signal is lowered.